It is well known to use particulates for extrusion products. In the conventional practice, the particulates may be preformed or pressed into billets by various processes such as hot- or cold-isostatic pressing and mechanical-die pressing, and the billets are then extruded. The results of these processes are billets that may be at substantially one hundred percent theoretical density, which are thereafter inserted in an extrusion press and extruded through a die having a cross-sectional shape in the aperture which is the mating form to the desired final product.
Another prior practice combines the steps of compressing the powder or particulate material in a press causing some compaction, then compressing further, and immediately extruding it, all in one continuous process.
It has been found that certain difficulties arise if one attempts to extrude directly from porous lightly compacted material into a final product.
U.S. Pat. No. 3,805,574--Wessel, shows apparatus which is intended to overcome the problems created by gas, usually air, trapped in the pores between the particulate during the compression and extrusion processes.
U.S. Pat. No. 4,059,896--Osa et al, reveals a two step process for converting aluminum scrap into finished articles in which the particulate aluminum scrap is first compacted to a density of between 1.8 and 2.4 g/cm.sup.3 to form a billet. In the process the trapped air is allowed to escape to the periphery of the billet after which it escapes in the second extrusion where the temperature is raised to between 350.degree. and 550.degree. C.
In U.S. Pat. No. 3,559,271--Nilsson, a process of hydrostatically compressing a lightly compacted powder/particulate billet is shown. The billet is compacted to a density of about sixty percent of the theoretically possible density; i.e., the density of a solid billet of the same material. In the process a temporary liquid tight plastic barrier of polyvinyl chloride or varnish is coated on the billet.
U.S. Pat. No. 4,087,893--Satt et al., discloses a process of producing a heat pipe in which a billet is prepared having an interior grooved surface. The entire internal cavity including the grooves is filled with a water soluble salt core. The billet is extruded through the die to reduce the size thereof and then the salt is dissolved and removed leaving a hollow heat pipe.
U.S. Pat. No. 2,123,416--Graham, reveals a method and apparatus for extruding powder and other particulate materials in which the powder or pieces of metal are held in a thin wall metal container which may completely enclose the powder or which may be open at the end. This patent shows the use of an extrusion die having an included angle greater than 90.degree..